This type of circuit breaker involves DC sensing, a function which is preferably carried out by a sensor placed within the housing of the circuit breaker. As shown in U.S. Pat. No. 4,743,875 of William J. Murphy, it is known to design a DC shunt so that it can be placed entirely within the confines of the housing of the circuit breaker.
A circuit breaker of the DC type is generally provided with one or more poles. Multiple poles, or contacts, are used in respective sections of the DC line so as to insure more reliability. Although the DC sensing function may be associated with one pole, it is known, from U.S. Pat. No. 4,492,941 of Harry H. Nagel, to place DC sensing devices on the respective such poles of a circuit breaker, the object of such parallelism being also to increase the overall reliability.
A circuit breaker generally includes a trip unit comprising: 1/ a section (INSTANTANEOUS) which responds instantaneously to any overcurrent of the sensed DC line exceeding a predetermined critical level, i.e. as require instantaneous protection, like at startup; 2/ a section (SHORT-TIME which tolerates high currents for short periods of time without tripping as prescribed by a short-time pickup level of the current sensed and generally predicated upon an inverse-time relationship of the sensed current; and 3/ a section (LONG-TIME) establishing a long-term delay, generally predicated on the square of the current.
See for instance: U.S. Pat. Nos. 4,266,259 and 4,423,458.
Tripping and DC sensing are functions which preferably have been placed within the housing of the circuit breaker. The afore-stated Murphy patent is illustrative of a DC sensing function designed so that it can be placed entirely within the housing of the circuit breaker, or contactor. So is the trip unit function preferably placed entirely within such common housing. Another function required on the circuit breaker is the power supply. A power supply and converter are also used to provide potential and current for the components of the circuitry. It is also desirable that such power supply and converter be placed within the housing and in proximity to the tripping unit. A problem arising from interconnecting power supply, sensing elements and the trip circuit is electrical isolation. In this regard, there are two components of electrical circuitry which are known to provide good electrical isolation. One is the transformer. It involves the transmission of power and current. The other is photo-electric coupling. The latter involves essentially the transmission of signals. The object of the present invention is to combine these two types of electrical isolation functions for providing an improved and more compact integration of the power supply, DC sensing and tripping unit functions within a circuit breaker and, more specifically, all within a common housing.